<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

</body>
<script>
    var time = prompt('请您输入一个0~59之间的数字');
    var result = time < 10 ? '0' + time : time;
    alert(result);
</script>

</html>